The present invention relates generally to methods for pyrolyzing carbonaceous material containing pyritic sulfur wherein the pyritic sulfur decomposes at pyrolysis temperatures to form hydrogen sulfide which is emitted with the gases and oil products resulting from the pyrolysis. More specifically, the present invention relates to a system and method for reducing the amount of hydrogen sulfide emitted during pyrolysis of these carbonaceous materials.
Oil shale is a composite material made up of marlstone-type minerals in which the organic polymer kerogen is intimately mixed. Vast reserves of oil shale are present in large areas of Utah, Wyoming and Colorado. Although the extent of these reserves is not definitely known, they have been estimated at being between three and seven trillion barrels. With such vast reserves of oil shale available, it is no wonder that numerous processes have been developed for converting this vast reserve of kerogen into a commercially useful form. As is well known, in order to convert the organic polymer kerogen into commercially useful shale oil, the kerogen must be decomposed and separated from the inorganic or mineral components of oil shale.
The majority of present processes for removing kerogen from oil shale involve heating the oil shale in a reducing atmosphere to pyrolyze the kerogen to form volatile hydrocarbon products. These products are separated from the inorganic portion of the oil shale and recovered by condensation and other conventional procedures.
The mineral or inorganic portion of oil shale includes major amounts of dolomite with lesser amounts of calcite, quartz, illite, albite and microcline also being present. In addition, the mineral portion of the oil shale will include pyrite in quantities ranging up to as much as five percent by weight. Typically however, pyrites are usually present in quantities on the order of one percent by weight. Pyrites are defined as any of the naturally-occuring metal sulfides which include iron disulfide, copper-iron disulfide and tin sulfide. Pyritic sulfur in oil shale is for the most part present as iron disulfide. Iron disulfide may occur in two crystalline forms, pyrite and marcasite.
Conventional oil shale pyrolysis processes involve heating the oil shale in a rotating retort or fluidized bed to temperatures in the range of 800.degree. F. to 1100.degree. F. A reducing atmosphere is maintained within the retort to prevent undesirable oxidation of the released shale oil products. Within the above pyrolysis temperature range, not only does kerogen undergo decomposition and volatilization, but the pyrites present in the oil shale also decompose to form hydrogen sulfide. Hydrogen sulfide is a highly toxic gas which is a well-known byproduct formed during treatment of sulfur containing carbonaceous materials. The hydrogen sulfide must be separated from the volatile products and either vented to the atmosphere, converted to a nontoxic form or otherwise processed to form a waste product which may be suitably disposed. Environmental pollution standards strictly limit the amount of hydrogen sulfide which may be vented to the atmosphere, so venting of the toxic gas is not possible. The other alternatives for preventing hydrogen sulfide emissions involve either processing the formed hydrogen sulfide to a nontoxic form or easily disposed non-polluting waste or alternatively preventing the formation of hydrogen sulfide entirely.
U.S. Pat. No. 3,960,513 discloses a process for reducing the amount of sulfur compounds emitted during coal combustion. This method involves converting the pyritic sulfur present in the coal to sulfate sulfur such as iron sulfate or ammonium sulfate. The coal which has been treated to convert pyritic sulfur to sulfate sulfur is then washed with water to dissolve and remove the water soluble sulfate sulfur from the coal. The wet coal is then dried and passed to a power plant for utilization. Although this process provides a technique for removing pyritic sulfur from carbonaceous material, it also includes the undesirable step of having to wash the sulfates from the coal. This produces a wet feed material which must be dried prior to pyrolysis. It would be desirable to provide a process which reduces the amount of pyrites decomposed during pyrolysis and emitted as hydrogen sulfide without the necessity for cumbersome and energy inefficient washing of the material prior to processing.